In general, magnetic recording/repoducing apparatus utilize the action of a magnetic head. During reproducing, the magnetic head scans a recording medium, records the signal data on the medium, forming magnetic tracks. The digital signals on the magnetic tracks can be of different categories. Depending on the different catagories, a magnetic track can be divided into a number of areas to record different kinds of signals. During reproducing, the magnetic head scans the original recording tracks, and reads the digital signals on these tracks.
In the foregoing recording/reproducing apparatus, there is, in addition to the normal recording and reproducing modes of operation, allowed another "after recording" mode. In the "after recording" mode when the magnetic head scans a recording track, recording action is performed on some of the divided areas while reproducing is performed on other areas. The function of this mode is to "overwite" certain areas with new signals and to retain the signals on some other areas.
In the "after recording" mode, the reproducing apparatus must accurately know the relative positions of the magnetic head and the magnetic track in order to accurately control the timing of the reproducing and the recording actions. In other words, recording action is performed in the "after recording" area, while reproducing action is performed in other areas. If the relative positions of the recording head and the magnetic track are not accurately aligned, the "after recording" area may not be completely, overwritten, while the area not intended for "after recording" may inadvertently be overwritten.
In conventional recording/reproducing apparatus, the relative positions of the recording head and the recording media cannot be accurately controlled. Furthermore, the design of the recording media format does not consider how the recording/reproducing apparatus can learn the relative positions of the magnetic head and the magnetic track. Thus, using conventional recording/reproducing apparatus and traditional magnetic track format for "after recording", one may find it difficult to control accurately the recording/reproducing function and to overwrite in the designated area.
The foregoing recording/reproducing apparatus is typical of a rotary-head digital audio tape recorder (R-DAT). In a R-DAT, a magnetic head A and a magnetic head B are fixed on a rotating magnetic drum. During rotation, the magnetic heads follow the rotation to scan the magnetic tape. The magnetic head record on the magnetic track, which is at an oblique angle with the running direction of the tape, or reproduce what is recorded on the magnetic tape. The track format of the R-DAT, as shown in FIG. 1, is composed of a SUB-1 area, an ATF-1 area, a PCM area, an ATF-2 area, a SUB-2 area, etc. and the IBG signals between these areas. The SUB-1 and SUB-2 areas are sub-areas for pack data recording. The PCM area is for recording an audio frequency signal after pulse-code modulation. The Sub-areas and PCM area are composed of data blocks. The block formats are shown in FIG. 2. There is a block synchronization signal at the beginning of each block. Following the block synchronization signal, the two bytes W1 and W2 consist of ID (identification) code, frame address and block address, as shown in FIG. 3. In FIG. 2(a), the byte following W1 and W2 is the parity byte of W1 and W2. The ATF-1 and ATF-2 areas are for recording auto track finding signals.
The "after recording" function of the R-DAT is to overwrite the sub-areas of the magnetic tape already occupied with recorded signals, and to maintain the status quo of the original signals in other areas. Alternatively, the PCM area is overwritten and remaining areas are unchanged. During "after recording", the changeover of the recording/reproducing action is traditionally controlled by two methods:
Method 1: Use a drum phase generator (DPG) signal as a reference to determine the relative positions of the magnetic head and the magnetic tape. The drawback of this method is that the accuracy depends on the mechanical parts and cannot be accurately controlled.
Method 2: Detect certain signals in the ATF areas as a reference to determine the relative positions of the magnetic head and the magnetic track. However, due to the low frequency of the ATF signals, even a few cycles of errors in measurement can cause large resultant error. Besides, the ATF signals have waveforms, and therefore cannot prevent any spurious ATF signals from acting falsely as a reference signal.
The preamble areas, post-amble areas and inter-block gap (IBG) areas, shown in FIG. 1, enclosing the data areas provide the tolerance for the accuracy of overwriting during after-recording using the conventional methods. Nevertheless, the mechanical specification is still limited for the method 1, or the spurious ATF signal still causes serious results when using the method 2.
The drawbacks of the prior art are illustrated, in U.S. Pat. No. 5,021,897 by Yoshino et al and U.S. Pat. No. 4,628,372 by Morisawa. The Yoshino patent corresponds to Method 1. The disadvantage of this method is that the accuracy for the relative position between the magnetic head and the magnetic tape must depend on the mechanical parts used, and cannot be accurately controlled. Morisawa disclosed a method for recording and detecting address code signals to separate program (audio) segments on a magnetic tape. The address code signals are only recorded in the unrecorded positions (blank or silent portions) between two individual program segments in order to distinguish two songs. In Yoshino and Morisawa, due to the low frequency of the ATF signals, even a few cycles of errors in measurement can cause large resultant error. Their ATF signals have waveforms and therefore cannot prevent any spurious ATF signals from acting falsely as a reference.